ABSTRACT
<b>[Background]</b> Although we have demonstrated the clinical effect of acupuncture therapy with epidermal stimulation by needle penetration, its neurophysiological mechanisms are unclear. To investigate the clinical effects of epidermal stimulation by needle penetration on muscle tone, we tested the Hoffmann's reflex (Hreflex) of the soleus muscle during epidermal needle stimulation. <br><b>[Methods]</b> Eighteen healthy participants were recruited. We tested the soleus H-reflex elicited by tibial nerve stimulation before and after epidermal stimulation by needle penetration. Epidermal stimulation was performed at several points on the Achilles tendon by needle penetration. We analyzed the amplitude ratio of H/M obtained from the waveforms of the soleus muscle before and after epidermal stimulation. <br><b>[Results]</b> The amplitude ratio of H/M was significantly decreased during stimulation when compared with ratio during rest (p < 0.05). Each participant demonstrated a decrease in soleus muscle amplitude ratio of H/M during epidermal stimulation by needle penetration as compared with the ratio during rest. <br><b>[Discussion]</b> Epidermal stimulation of the muscle by needle penetration activated the inhibitory interneurons of the dermatome at the stimulation site. Therefore, epidermal stimulation by needle penetration was considered as a possible method to induce muscle relaxation. <br><b>[Conclusion]</b> It is suggested that epidermal stimulation by needle penetration for 2 minutes for the insertion of the Achilles tendon results in an inhibitory effect by stimulating the spinal cord neural mechanisms that correspond to the soleus muscle.